1. Field of the Invention
The present invention relates to a data sequence generator for generating a data sequence suitable for transmission of a data sequence composed of variable length frames having information data of variable length particularly in an environment where a code error is liable to occur, a transmitter using the data sequence generator, an information data decoder for decoding information data from the data sequence, a receiver using the information data decoder, a transmitter-receiver using the data sequence generator and the information data decoder, a data sequence generating method for generating the data sequence, an information data decoding method for extracting information data from the data sequence, and a recording medium recording the data sequence.
2. Description of the Related Art
In case of transmitting a data sequence having a frame structure, detection of the synchronous position of a frame in a received signal sequence, namely, a frame synchronization is very important, and particularly in transmission of a variable length frame, since the synchronous position varies with each frame, a more highly accurate frame synchronization is required. And header information necessary for decoding data (information data) in an information data portion is also as important as the frame synchronization, and protection of the header information from a code error is also required.
As a method for realizing a frame synchronization (frame synchronization method), a method for realizing a frame synchronization by adding a unique word (UW: specific information) such as the M sequence and the like to each frame as a frame synchronization code in a transmitter side, and detecting the unique word from a received signal sequence in a receiver side (hereinafter referred to as a first frame synchronization method) is widely known. The first frame synchronization method does not limit in particular the position where a unique word is to be added, but a variable length data transmission system adopting the synchronization method often adds a unique word usually to the forefront of a frame, as shown in FIG. 1A, in order to simplify the process.
By the way, in the first frame synchronization method, when a unique word and information data in a frame coincide with each other in a unique word detecting process, an xe2x80x9cerroneous synchronizationxe2x80x9d occurs which is a phenomenon in which an erroneous position results in being adopted as a frame-synchronous position. And in a unique word detecting process, when the unique word in a frame contains an error which has occurred on a transmission channel (communication channel), a xe2x80x9cnon-detectionxe2x80x9d occurs which is a phenomenon in which the unique word cannot be detected from the frame and the synchronous position cannot be determined. Both the above-mentioned phenomena each are a frame synchronization error of a kind, and when such a frame synchronization error occurs, the receiver side cannot decode the whole frame for which a correct synchronous position has not been obtained and the frame comes to be discarded. Since the discard of a frame brings about a great reduction in data transfer rate, it is desired to reduce the frame synchronization error rate in the first frame synchronization method. Particularly, since the probability of occurrence of a code error is very high in a radio transmission channel and the like, it is desirable to adopting a frame synchronization method more robust against error.
Up to now, as a method for reducing the probability of occurrence of an erroneous synchronization (hereinafter referred to as a synchronization error rate), a method of using the result of error detection of header information is known. In a frame synchronization method adopting this method (hereinafter referred to as a second frame synchronization method), when a unique word is detected in a receiver side, then an error detection decoding process of header information is performed and if the header information does not have an error, a position where the unique word has been detected is determined as a synchronous position. That is to say, since the system fails in detection of header information even if a unique word and information data in a frame coincide with each other, it is possible to avoid an erroneous synchronization which has occurred in such a case. However, in an environment where a burst error occurs (burst error environment), the probability of occurrence of non-detection cannot be reduced even by using the second frame synchronization method. The reason is that it is impossible to detect a synchronous position in case that the unique word and header information in a frame are contained in a burst error section.
As a frame synchronization method for realizing a high-accuracy frame synchronization even in a burst error environment, there is a method which disposes frame length information in a frame as shown in FIG. 1B and detects a frame synchronization using this frame length information (hereinafter referred to as a third frame synchronization method). Since the frame length information coincides with the length from the position of the unique word of the frame to the position of the unique word of the next frame, according to the third frame synchronization method, it is possible to know in advance the position of the unique word in the next frame using the frame length information and reduce the probability of non-detection and erroneous detection of a unique word.
In the third frame synchronization method using frame length information, since the length of a frame nxe2x88x921 is determined if the header information and frame length information of the frame nxe2x88x921 shown in FIG. 2 can be correctly decoded, decoding of the frame nxe2x88x921 can be started without performing search of the next synchronous position (hereinafter referred to as a synchronization search). And since a synchronization search can be started at the forefront of the next frame, it is possible to reduce the probability of occurrence of an erroneous synchronization which is caused by erroneous detection of a unique word in an information data portion. In case of being not able to decode frame length information due to a burst error which has occurred over the synchronous position indicated by the frame length information in the frame nxe2x88x921 as shown in the frame n, it is possible to determine a section of the frame n and take out the frame n by searching a unique word from an appropriate position (position where there can be the unique word of the frame n+1) and detecting the unique word of the frame n+1 at a correct position. Therefore, by using jointly this method and the above-mentioned second frame synchronization method using header information (a method of reducing erroneous synchronization), it is possible to realize a high-accuracy frame synchronization even in a burst error environment.
By the way, in a data transmission having a frame structure, a frame to which header information necessary for decoding information data has been added is transmitted in many cases, and in the above-mentioned second frame synchronization method it is assumed that the header information is used. As an example of a data transmission procedure for adding header information, the HDLC (High Level Data Link Control) procedure being generally used in data transmission can be mentioned. In the HDLC procedure, in case that header information cannot be correctly decoded due to a transmission channel error, the whole frame cannot be decoded in the same way as the case that a frame synchronization error has occurred. Therefore, it is necessary to strengthen protection of header information to the same degree as the frame synchronization.
As a method for protecting header information, a method of error-correction-encoding the header information is conceivable. However, in case that an error exceeding the error correcting ability of the header information has occurred in the header information portion in a frame as shown in FIG. 2, even if the header information has been error-correction-encoded, since although a frame-synchronous position can be determined as described above the header information cannot be decoded, the information data cannot be decoded and the frame results in being discarded. That is to say, the whole error resiliency of a frame has been lowered due to difference in error resilience between the frame synchronization and the header information.
In order to improve the whole error resiliency of a frame it is conceivable to apply an error correction code having a high error correction ability, but in a burst error environment a robust error correction ability coping with densely occurring errors becomes necessary and the redundancy of transmission data results in being greatly increased.
And as a method for efficiently protecting data from a burst error, a method which separates the data in point of time and transmits them together with error detection information repeatedly at plural times is effective. This method is described in detail, for example, in xe2x80x9cA Very Low Rate Moving Image Encoding Method Having a Robust Error Resiliency (2)xe2x80x94Duplication of Important Information and Reversible Codexe2x80x94, D-244, 1996 General Convention of The Institute of Electronics, Information and Communication Engineersxe2x80x9d. However, this method requires that the positions of data transmitted separately at plural times have been determined in advance, and it cannot be applied to transmission of such data whose position cannot be determined as the header information in a variable length frame. And in a random error environment having a high code error rate, when an error occurs in any data separately and repeatedly sent at plural times even if those errors are small in number, it is no longer possible to normally receive the data. Although it is of course conceivable to error-correction-code each of the data to be sent separately and repeatedly at plural times, when attempting to obtain a sufficient error correction ability, eventually the redundancy of transmission data becomes great and the transmission efficiency of data is greatly deteriorated in cooperation with the redundancy of sending separately and repeatedly the data at plural times.
As described above, according to a variable length data transmission system adopting a former frame synchronization method, it has been difficult to satisfactorily perform at the same time the synchronization of a variable length frame and the protection of such important information as header information and the like in a transmission channel having a high code error rate.
An object of the present invention is to provide a data sequence generator and an information data decoder which are capable of satisfactorily performing at the same time establishment of synchronization of variable length frames and protection of header information in a variable length frame as suppressing increase of the redundancy of transmission data in an environment where a burst error and a random code error can occur, a transmitter using the data sequence generator, a receiver using the information data decoder, a transmitter-receiver using the data sequence generator and the information data decoder, a data sequence generating method, an information data decoding method, and a recording medium.
In order to attain the above-mentioned object, a data sequence generator according to the present invention is a data sequence generator for generating a data sequence to be transmitted through a transmission channel by generating according to input of information data a variable length frame having a first storage area storing variable length information data inputted from the outside, a second storage area storing specific information for establishing a frame synchronization, a third storage area storing header information necessary for decoding the information data and establishing the frame synchronization, and a fourth storage area storing frame length information indicating a frame length, the data sequence generator being characterized by comprising a header information error detection encoding means for obtaining header information error-detection-encoded data by applying an error detection encoding process to the header information, a header information storing means for storing the header information error-detection-encoded data into the third storage area corresponding to the header information, and an inserting means for inserting the header information error detection encoding data in an insertion position which is a position to be determined from the timing of the frame synchronization in the data sequence and is separated from the third storage area.
And an information data decoder according to the present invention is an information data decoder for decoding the information data from a data sequence generated by the data sequence generator, the information data decoder being characterized by comprising a first header information error detection decoding means for performing an error detection decoding process on data stored in the third storage area of the variable length frame being under decoding and for outputting a first error detection result and a first decoding result, an insertion position determining means for determining the insertion position corresponding to the information data stored in the first storage area of the variable length frame on the basis of the timing of the frame synchronization of the variable length frame, a second header information error detection decoding means for performing an error detection decoding process on data inserted in the insertion position and for outputting a second error detection result and a second decoding result, a header information selecting means for selecting one decoding result containing no error out of the first decoding result and the second decoding result on the basis of at least one of the first error detection result and the second error detection result, and an information data decoding means for decoding the information data using the decoding result selected by the header information selecting means.
According to the above-mentioned composition, when a frame synchronization is established it is possible to detect plural pieces of header information for a piece of information data. Since these plural pieces of header information are arranged separately from one another, the possibility that an error occurs in every piece of header information is low even if a burst error occurs in a transmission channel. Additionally, since each header information has been error-detection-encoded, the information data decoder can easily determine the header information having no error. That is to say, according to the above-mentioned composition, it is possible to realize establishment of a variable length frame synchronization and protection of header information in a sufficiently high quality without greatly increasing the redundancy even in an environment where a burst error and a random error can occur.
And an error detection process may be performed on frame length information, and in this case it is possible to use together a frame synchronization using frame length and a frame synchronization using specific information and header information. Therefore, a frame synchronization can be established in higher accuracy. Furthermore, since an error detection process is performed on the header information used in establishment of a frame synchronization, it is possible to more improve the accuracy of the frame synchronization. Moreover, if performing an error detection and error correction process on header information to be used in establishment of a frame synchronization and performing only an error detection process on the other header information, it is possible to reduce the probability of occurrence of non-detection in a random error environment as suppressing increase of the redundancy. Of course, an error detection and error correction process may be performed on header information not to be used in establishment of the frame synchronization, or the reliability of frame length information may be improved by performing an error detection process or an error detection and error correction process on the frame length information.
And in order to attain the above-mentioned object, a transmitter, a receiver and a transmitter-receiver according to the present invention are respectively provided with a data sequence generator, an information data decoder, and a data sequence generator and an information data decoder. A communication system provided with at least two out of these transmitter, receiver, and transmitter-receiver can realize establishment of a variable length frame synchronization and protection of header information in a sufficiently high quality as suppressing increase of the redundancy even in an environment where a burst error and a random error can occur.
And a data sequence generating method according to the present invention is a data sequence generating method for generating a data sequence composed of a variable length frame having a first storage area storing variable length information data, a second storage area storing specific information for establishing a frame synchronization, a third storage area storing header information necessary for decoding the information data and establishing the frame synchronization, and a fourth storage area storing frame length information indicating a frame length, the data sequence generating method being characterized by performing for every at least one piece of the information data a storing step of error-detection-encoding the header information corresponding to the information data and storing it into the third storage area corresponding to the information data, and an inserting step of error-detection-encoding the header information and inserting it in a position which is a position to be determined from the timing of the frame synchronization in the data sequence and is separated from the storage area.
And in order to attain the above-mentioned object, an information data decoding method according to the present invention is an information data decoding method for decoding the information data from a data sequence composed of a variable length frame having a first storage area storing variable length information data, a second storage area storing specific information for establishing a frame synchronization, a third storage area storing header information necessary for decoding the information data and establishing the frame synchronization, and a fourth storage area storing frame length information indicating a frame length, the information data decoding method being characterized by performing for every at least one of the variable length frames a first error detection step of error-detection-decoding data stored in the third storage area of the variable length frame being under decoding, a second error detection step of error-detection-decoding data inserted in a position determined from the timing of the frame synchronization, and a decoding step of decoding the information data stored in the first storage area of the variable length frame using one decoding result countering no error out of a decoding result in the first error detection step and a decoding result in the second error detection step.
According to the above-mentioned method, from the same reason as described above, it is possible to realize establishment of a variable length frame synchronization and protection of header information in a sufficiently high quality without greatly increasing the redundancy even in an environment where a burst error and a random error can occur.
And in order to attain the above-mentioned object, a recording medium according to the present invention is a recording medium which records a data sequence composed of a variable length frame having a first storage area storing variable length information data, a second storage area storing specific information for establishing a frame synchronization, a third storage area storing header information necessary for decoding the information data and establishing the frame synchronization, and a fourth storage area storing frame length information indicating a frame length, and is sequentially accessed at a specified speed, wherein the variable length frame has a fifth storage area storing the header information at a position which is a position to be determined from the timing of the frame synchronization in the data sequence and is separated by a distance based on the speed from the third storage area.
In a system composed of this recording medium and a device for accessing the recording medium, from the same reason as described above, it is possible to realize establishment of a variable length frame synchronization and protection of header information in a sufficiently high quality without greatly increasing the redundancy even in an environment where a burst error and a random error can occur.